Method and system for communicating intelligence



Sept. 3, 1946.

METHOD AND SYSTEM FOR COIVHVIUNICATING INTELLIGENCE N. H. YOUNG, JR

` Filed Sept. 19, 1942 INVENTOR /VOPMN H. YOUNG, JR.

BY I D M.

. ATroRNl-:Y

Patented Sept. 3, 1946 VUNITED STATES PATENT OFFICE METHOD AND SYSTEM FOR COMBIUNICAT- ING INTELLIGENCE Norman H. Young, Jr., Jackson Heights, N. Y., assignor to Federal Telephone and Radio Corporation, a corporation of Delaware Y Application September 19, 1942, Serial No. 458,934

8 Claims.

This invention relates to a new method and;

system for communicating intelligence, and is particularly adapted for use as a secret radio communicating system.

l Accordingly, an object of this invention is to provide a novel and simple method and means for communicating intelligence secretly.

`Another' object of this invention lies in providing a novel method and means for transmitting intelligence which can be intelligibly received only upon a properly correlated receiver.

Other objects and advantages of the invention will become apparent from the following description of a preferred form of system, illustrated in the accompanying drawing, in which:

Fig. 1 is a block circuit diagram of a complete transmission system in accordance with the present invention; and

Fig. 2 is a diagram illustrating the operation of the system by means of correlated curves.

Referring specifically to Fig. 1, and more particularly to the transmitting station I0, the latter will be provided with a constant frequency source such as a pilot oscillator I2, the output of which is passed through a frequency multiplier I6 to produce oscillations at a harmonic frequency. The frequency multiplier I6 is preferably a frequency tripler of some suitable well-known construction. The outputl from the frequency multiplier I6 passes through a suitable phase modulator 22.

At ,I8 I have provided a microphone as an eX- ample of a suitable intelligence translating device, the output of the microphone I8 being preferably amplified by audio amplifier 20 and applied to the phase modulator 22 to phase modulate the harmonic oscillations received from the frequency multiplier IE.

In a suitable mixer 26 the phase modulated harmonics from the phase modulator 22 are added to the fundamental wave derived from the pilot oscillator I2 to produce, by simple addition, a resulting wave which will vary slightly in accordance with the amount of phase deviation of the harmonic component. This resulting wave is impressed upon a modulator 28 connected between a suitable carrier wave source 32 and the transmitter 30. The resultant modulated carrier wave will then be radiated by antenna means 34.

The receiving station generally indicated at 35 will include an antenna 36 connected to a receiver 38, which will include suitable demodulating means for demodulating the basic carrier in accordance with the type of modulation employed at modulator 28 of the transmitting sta- 2 tion I0. The output of the receiver 38 is connected, on theone hand, to a pilot filter 40 which is sharply tuned t0 the frequency 0f the pilot oscillator I2 a1; the transmitting station and to a harmonic filter 44 tuned and employed to extract the phase modulated harmonic component. The pilot filter 40 is used at the receiver 35 as a local pilot frequency generator, and the output therefrom passes through a frequency multiplier i2 comparable to the frequency multiplier IG at the transmitting station. By means of a balanced detector 46 the phase-modulated harmonic derived from the filter 44 and the unmodulated harmonic derived from the frequency multiplier 42 are compared and the diierence in phase is translated into suitable audio frequency signals which, through an amplifier 4I are impressed upon the loud speaker 50. The balanced detector 4:6 used for demodulation may be of the type known in the television art as a discriminator such as described, for example, in the book Principles of Television Engineering" by Donald G. Fink, page 405, Fig. 250, published by McGraw- Hill Book Company, Inc. 1940.

It is believed that the operation of the aforedescribed system will be clear to those skilled in this art. It involves the principle of modulating a carrier wave as for example, by amplitude modulation or frequency modulation, by means of a wave which is a-combination of a fundamental wave such, for example, as a 10 kc. wave and the phase-modulated harmonic thereof, such as a phase-modulated 30 kc. wave. The resultant signal or intelligence which is transmitted will be diflicult of detection and can only be reconverted into an intelligible signal provided the receiver is equipped with means for providing a source of frequency exactly the same as that provided at the transmitter and with the proper accurately designed harmonic filter. If the frequency multiplier I at the transmitter also reverses the phase of the harmonic by the modulation of the carrier wave in accordance with the transmitted intelligence will be most difficult to detect by unauthorized receivers, as the resultant voltage change in the modulating wave will be extremely slight. This may be graphically illustrated by way of example in Fig. 2, in which the wave A may represent the fundamental frequency produced by the pilotI oscillator I2 while B represents the third harmonic with a 180 phase shift. The resultant wave produced by the addition of waves A and B when the wave B is exactly 180 out of phase with the wave A, is represented by the curve C. The phase modulation of the harmonic B which, in most cases, will not exceed 30 of the harmonic or 10 of the fundamental will produce a wave C which, it will be noted, will differ only slightly in amplitude from the wave C. The Wave C is the resultant of the addition of the fundamental A to the harmonic B' shifted 30 relatively to the wave B. It will be noted that by shifting the harmonic 180"V with respect to the fundamental A the peak of the harmonic substantially coincides with the peak of the fundamental thus producing sharply de-1 fined peaks in the resultant wave. As previously mentioned,'however, the peaks of the resultant wave will differ only slightly in amplitude for relatively large phase shifts in the harmonic. Preferably, the amplitude of the harmonic Will be approximately no greater than 30% Of the amplitude of the fundamental. It is to be noted that if the frequency multiplier I6 at the transmitting station includes means to shift the phase of .the harmonic 180, similar means must be include'd in the frequency multiplier 42 at the receiving station.

vlt will be clear to those skilled in this art 'that various changes may be 'made in the afcredescribed 'system Without departing from the princples of thev present invention. As has been previously mentioned, the manner in which the carrier-'wave is 'modulated by the resultant phase modulated wave is unimportant. The manner in which the pilot frequency Vand harmonic are generated `is likewise of no particular importance. instead of phase modulating the harmonic by translated audible signals, the source of intelligence can be of any type which will suitably operate the phase modulator v22. Changes in the 'details of the transmitter l0 will also result in comparable Achanges in details of the' vapparatus used at 'the receiver 35. It is also obvious that as far 'as the basic principles of this invention are concerned, it is possible to phase modulate the lower frequency wave instead of the higher 'frequency wave provided, of course, the yfrequencies are harmonically related. l

While the system according to my invention is particularly adapted for use in radio Ycommunication, it will also be clear that a direct line connection between the transmitting stationk and the receiving stationcan be used, if desired.

Accordingly, while for the purposes of ldescribing the invention, reference hasbeen made to certain specic devices and'relationships, it is to be understood that the invention is not limited thereto except as maybe required 'by the claims which follow.

-What I claim is:

A transmission system comprising, in comceiver, a frequency multiplier connected to said pilot frequency filter, a balanced detector fed from said harmonic filter and said frequency multiplier, and audio output means connected to said balanced detector.

2. The combination according to claim 1, in which said frequency multipliers include means for shifting the phase of the derived harmonic 180.

3. The method of transmitting and receiving intelligence'which includes the steps of phase modulating a harmonic of a predetermined pilot frequency in accordance with audio signals, addving 'the phase-modulated harmonic and the 'fundamental pilot frequency together, transmitting the wave resulting fromrthis addition, receiving Vsaid wave, separating the pilot frequency and harmonic, changing the filtered pilot frequency to the samefrequency as the harmonic, comparing the phase deviation between the filtered harmonic and the derived harmonicLand r'erno'dlicing the `audio signals in laccordance with said phase deviation.

4. A transmitting station including, in combination, a transmitter, a source of carrier Waves, a modulator connected between said transmitter and carrier wave source, a iirst constant frequency source, a second constant frequency source producing a frequency which is a harmonic y ceiver producing the unmodulated harmonic,

loination atransrnitting station 'including a transmitter, a source of carrier waves, a modulator 'connected between said transmitter and carrier wave source, a 'pilot oscillator, a frequency multiplier connected to said pilot oscillator, a phase modulator fed by said frequency multiplier, a mixer connected to saidvcarr'ier modulator, means for If'ee'ding'the output of sai-'d` pilot oscillator and said phase modulator to said mixer, and an audio frequency source connected to said phasemodulater, and a receiving station including a receiver adapted toreceive signals from said transmitter, apilot frequency filter and a harmonic frequency filterboth connected to the outputof said remeans forV comparing ksaid phase-modulated harmonic with; said locally generated harmonic, and means'ifor reproducing the transmitted signal in accordance with the differences in phase between said harmonics.' l n y675A transmitting station including, in combination, a transmitter, a lsource ofl carrier waves,` a modulator connected between said transmitter and carrier wave source, a constant frequency source, 'a frequency Vmultiplier Iconnected Vto said constant frequency source, a phase modulator Aconnect/ed 'to said frequency multiplier, a mixer connected to said carrier wave modulator, means for feeding the output 4of vsaid constant frequencyY source and of said phase modulatorjto said mixer, and a variable frequency signal source operatively connected -to said phase modulator.

Y"7. The combination vaccording, to claim lo, in which lsaid frequency multiplier includes means for shifting 'the phase of 'the harmonic 180".

8. The method of signaltransmission, which includes the steps :of lphase modulating a waveV of frequency -harinonically related to 'a predeter-- mined 'pilot frequency in Yaccordance with the signals to betlansmitted, adding the phase modulated frequen cy to the pilot frequency, and moduf lating a carrier 'waveby the wave resulting from this addition. Y

' NORMAN H. YOUNG, J R. 

